1. Field of the Invention
The present invention relates to a method for handling wellhead structures, and especially to a method for transporting underwater wellhead assemblies to an offshore jack-up drilling rig or oil-production platform and installing it on the ocean floor.
2. Description of the Prior Art
In recent years it has become desirable to use offshore jack-up drilling rigs or barges from which to drill wells in marine locations. The jack-up drilling rig consists of a buoyant platform which is provided with a number of legs extending through the platform vertically and can be moved up and down in a vertical line by jacking or lifting systems consisting of the jack arranged on a longitudinal side of the leg and the pinion mounted on the platform to engage with the pinion. In the rig of this type which will be referred to as a "jack-up-rig", the legs are lifted up out of the surface of the water and the buoyant platform is usually towed to a preselected offshore drilling location by, for example, a tug boat or boats, the legs are then lowered down to the ocean floor and the platform is raised up above the surface of a body of water so that it can be fixed and supported on the ocean floor. When carrying on a multi-well drilling operations in the ocean, a subsea template having a plurality of receptacles therein is set on the ocean floor, and holes are drilled through some of these receptacles. In drilling oil and gas wells in the ocean floor in depths up to approximately 90 meters, the jack-up-rig may usually be operated by using a subsea wellhead assembly and a surface blowout preventer extending above the surface of a body of water for safety and reliability. However, in drilling the ocean floor in depths of water greater than about 120 meters or under hostile environmental conditions such as strong wind and severe sea conditions, the jack-up-rig must employ a subsea blowout preventer and a subsea wellhead assembly. In practice, these wellhead assemblies are positioned in depths of water greater than the depth at which a diver can safely and readily work. Furthermore, the base member which is a horizontally-extending frame called a "template" is for example, about 24 meters in length, about 6 meters in width and more than 140 tons in weight. Such a heavy and cumbersome structure is impossible to carry or transport on the vessel type or semi-submersible drilling barge.
Consequently, the template has been handled by, for example, crane barges or derrick cargo barges separate from the drilling barge. This is very expensive and requires a number of means for lowering the template down to the ocean floor and bringing the drilling barge to its correct position with respect to the anchored template.